In July 1997, the U.S.
EPA decided that the 8-hour primary ozone standard would protect
vegetation. Based on sound scientific research, the EPA earlier
had discussed the possibility that a new standard, specifically
designed to protect crops and trees, might be appropriate. Two
exposure indices, the SUM06 and W126, were discussed in detail.
The SUM06 cumulative index, using a threshold value, added all
of the hourly average concentrations greater than or equal to
0.06 ppm. TheW126 exposure index, while focusing on all hourly average
concentrations, provided greater weight to the higher hourly
average concentrations than the mid- and lower- range values.
In January 1996, a group of key vegetation effects scientists
met in North Carolina at a workshop
to address the form
and level of a possible secondary standard. At that meeting,
9 of the 14 experts endorsed the W126 exposure index as the index
of choice. The scientific background for the exposure index can
be found in the EPA's release of the 1996 Ozone Staff Paper.
A.S.L. & Associates' reviewed the document
and identified key points. In October 1999, the U.S. Department
of Interior and the U.S. Department of Agriculture proposed using
the W126 exposure index accumulated over a 24-hour period, combined
with the number of hourly average concentrations equal to and
greater than 0.10 ppm (i.e., N100), to protect vegetation from
injury and damage.

The N100 index was included
with the W126 exposure index because the exposure-response equations
used to determine the level of the W126 were based on the National
Crop Loss Assessment Network (NCLAN) experimental protocol. This
protocol included the fumigation of vegetation with frequent
numbers of hourly average concentrations greater than or equal
to 0.10 ppm. The N100 exposure metric takes into consideration
the large number of hourly average concentrations used in the
NCLAN fumigation protocol. The use of the N100 exposure index
is not a minor adjustment but a major adjustment
required for use with the W126 exposure index if NCLAN-type artificial
fumigation protocols are used to define the protective number
for the W126 index. If the N100 exposure index is not used in
combination with the W126 exposure index, the W126 will provide
inconsistent predictions for vegetation injury (e.g., spots on
plants) and damage (e.g., growth loss).

In 2007, the EPA's Ozone
Staff Paper (EPA, 2007) recommended that the W126 exposure index
be considered as a possible secondary ozone standard. In March
2007, EPA's Clean Air Scientific Advisory Committee (CASAC) recommended
that the W126 be adopted as a standard to protect vegetation
from ozone exposure. In June 2007, the EPA Administrator proposed
the W126 exposure index as a secondary ozone standard. On March
12, 2008, the EPA Administrator made the final decision on the
human health and vegetation ozone standards. EPA revised the
8-hour "primary" ozone standard, designed to protect
public health, to a level of 0.075 parts per million (ppm). The
previous standard, set in 1997, was 0.08 ppm. EPA decided not
to adopt the W126 exposure index. Although the EPA Administrator
recommended the W126 as the secondary ozone standard, based on
advice from the White
House (Washington
Post, April 8, 2008; Page D02), the EPA Administrator made the
secondary ozone standard the same as the primary 8-hour average
standard (0.075 ppm).

In May 27, 2008, health
and environmental organizations filed a lawsuit arguing that
the EPA failed to protect public health and the environment when
it issued in March 2008 new ozone standards. On March 10, 2009,
the US EPA requested that the Court vacate the existing briefing
schedule and hold the consolidated cases in abeyance. EPA requested
the extension to allow time for appropriate EPA officials that
are appointed by the new Administration to review the Ozone NAAQS
Rule to determine whether the standards established in the Ozone
NAAQS Rule should be maintained, modified, or otherwise reconsidered.
EPA further requested that it be directed to notify the Court
and the Parties within 180 days of the Court's order vacating
the briefing schedule of the actions the Agency has taken or
intends to take, if any, with regard to the Ozone NAAQS Rule,
and the anticipated time frame for any such actions.

On September 16, 2009,
the EPA announced it would reconsider the 2008 national ambient
air quality standards (NAAQS) for ground-level ozone for both
human health and environmental effects. The Agency planned to
propose any needed revisions to the ozone standards by December
2009 and issue a final decision by August 2010. On January 7,
2010, the EPA announced on its web site its proposal to strengthen
the national ambient air quality standards for ground-level ozone.
The EPA's proposal decreased the 8-hour primary ozone
standard level, designed to protect public health, to a level
within the range of 0.060-0.070 parts per million (ppm). EPA
proposed to establish a distinct cumulative, seasonal secondary
standard, referred to as the W126 index, which was designed to protect
sensitive vegetation and ecosystems, including forests, parks,
wildlife refuges, and wilderness areas. EPA proposed to set the
level of the W126 secondary standard within the range
of 7-15 ppm-hours. The proposed revisions resulted from a reconsideration
of the identical primary and secondary ozone standards set at
0.075 ppm in March 2008. On August 20, the Agency announced that
it would delay its final announcement to on or around the end
of October. In early November, the EPA announced that it would
reach a final decision on the ozone standards by December 31,
2010. On December 8, the EPA announced that it would delay its
final decision on the ozone standards until July 2011. EPA announced
on July 26 that it would not make a decision on the ozone standards
by its previously announced deadline of July 29. On September
2, 2011, President Obama requested that the EPA withdraw its
proposed revisions of the ozone standards.

On
November 26, 2014, the EPA Administrator announced that she is
proposing an ozone human health (primary) standard in the range
of 65 to 70 ppb and will take comment on a standard as low as
60 ppb. For the welfare (secondary) ozone standard, she is proposing
that the standard be the same as the health standard IF the final
health standard is set in the range of 65 to 70 ppb. The rationale
for the EPA proposal can be found at the EPA website. The Administrator believes that a health
standard in this range would protect vegetation from ozone exposures
of W126 values within the range of 13-17 ppm-h. She is also taking
comment on setting a W126 value in the range of 7-13 ppm-h, which
implies that she is still considering establishing a secondary
standard separate in form from the human health 8-h standard.
In August 2014, the EPA Staff recommended to the Administrator
that she select the ozone primary standard at a specific level
between 60-to-70-parts-per-billion. For the secondary standard,
the EPA Staff recommended that the Administrator establish a
3-month, 12-h W126 secondary standard, which
would have a specific value within the range of 7 to 17 ppm-h.
In October 2015, the Administrator set the human health and vegetation
standards at 0.070 ppm. The W126 exposure index continues to
be the exposure metric to assess vegetation effects. The administrator
is using the 0.070 8-h standard to control the W126 exposure
index to below values of 17 ppm-h.

For up-to-date information
on the W126 and other aspects of air pollution environmental
and human health effects information, please visit our News and Views section. Should you wish to learn
more about the science associated with assessing the importance
of peak ozone concentrations and how the peaks relate to vegetation
uptake and detoxification, please click here.

References

U.S. Environmental Protection
Agency (2007) Review of the National Ambient Air Quality Standards
for Ozone: Policy Assessment of Scientific and Technical Information
OAQPS Staff Paper. Research Triangle Park, NC: Office of Air
Quality and Planning and Standards, EPA-452/R-07-003. January.

Washington Post (2008)
It's Not a Backroom Deal If the Call Is Made in the Oval Office
by Cindy Skrzycki. Tuesday, April 8, 2008; Page D02.